Runners have a contagious energy, an unusually positive outlook on life, and let's face it -- they look good in their spandex shorts. But, according to dentists and health professionals, runners and other endurance athletes are more prone to tooth decay and dental problems than the rest of the population.

There is no requirement for people to cheer others on they don't know, but you see it happening along the whole of the race course. I observed the best of what we call "human." It affirmed for me that within humans reside compassion, generosity, thoughtfulness, and concern for others.

"I often come up with theorems in dreams, or while swimming or walking," says Schneider. "I once thought of a heuristic proof of the Prime Number Theorem on a walk, when I saw spring flowers falling froma tree."

Duffy's story offers a perfect example of a profound, but often overlooked truth about human toughness: Even the most physically punishing feats of endurance are less about the body than about the mind.